With an increase in the Internet traffic, there is a demand for a further increase in capacity of trunk transmission systems. As a technique for achieving a further increase in capacity of trunk transmission systems, a coherent transmission technique is attracting attention.
In a normal coherent transmission, when the polarization of an LO light source (local oscillator) of a receiver does not match the polarization of an input signal, it is difficult to appropriately receive the signal. For this reason, an optical device, such as a polarization stabilizer, is required to match the polarizations.
On the other hand, in a digital coherent transmission (for example, Patent Literature 1 and 2), a polarization separation function using a digital signal processing algorithm is used. In the digital coherent transmission, a polarization separation digital signal processing algorithm, such as CMA (Constant Modulus Algorithm) (Non Patent Literature 1 to 3), which separates two orthogonal signals (polarizations), is used to thereby make it possible to separate two polarization modes. By using this polarization separation function, the polarization multiplexing transmission in which different signals are transmitted in two orthogonal polarization modes can be achieved.